This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-049569, filed Feb. 25, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a voltage controlled oscillator.
The construction of a conventional voltage controlled oscillator is shown in FIG. 1. In the conventional voltage controlled oscillator, a control voltage Vctrl for controlling the oscillation frequency is used only for controlling the variable capacitance of a tank circuit 101 including a spiral inductor L and variable capacitance diodes (variable capacitance elements) D1 and D2 formed in the same semiconductor substrate.
The oscillation frequency of the voltage controlled oscillator is determined by the following equation (1).
fosc=1/(2xcfx80(LC)xc2xd)xe2x80x83xe2x80x83(1)
In the equation (1), L denotes the inductance of the spiral inductor L and C denotes the total sum of the capacitances of the variable capacitors C1, C2 and the parasitic capacitance associated with the whole elements constructing the voltage controlled oscillator.
In the voltage controlled oscillator, it is necessary to change the oscillation frequency according to the control voltage vctrl. Therefore, in FIG. 1, the intermediate potential of a node of the variable diodes D1 and D2 is controlled by the control voltage Vctrl to change the junction capacitances of the variable capacitors D1 and D2, thereby making it possible to attain a desired oscillation frequency.
As the characteristic of the voltage controlled oscillator, the oscillation frequency, variable frequency range and phase noise which is a parameter indicating the fineness of the oscillating frequency are very important.
The precision of the oscillation frequency is determined by the precision of the inductor L and the variable capacitors C1 and C2. The variable frequency range is determined by the capacitance variable range of the variable capacitors C1, C2 and the parasitic capacitance of the whole elements constructing the voltage controlled oscillator. The phase noise is determined by the parasitic resistance of the whole elements constructing the voltage controlled oscillator and a current Icc flowing in an amplifier 103.
The phase noise in a range in which thermal noise exists as a main component thereof is expressed by the following equation (2).
L(foffset)=kTReff(1+A)(fosc/foffset)2/Vrms2xe2x80x83xe2x80x83(2)
In the equation (2), k indicates the Boltzmann""s constant, T indicates the absolute temperature, Reff indicates the total sum of parasitic resistances of the whole elements constructing the voltage controlled oscillator, fosc indicates the oscillation frequency, and foffset indicates an offset frequency used for observing the phase noise and corresponding to a frequency offset from the oscillation frequency fosc. Vrms indicates an output amplitude of the oscillator.
The parameter A in the equation (2) is expressed by the following equation (3).
A=Gm, amp/Gnegxe2x80x83xe2x80x83(3)
That is, the ratio of energy Gm,amp consumed in the whole portion of the amplifier 103 to energy Gneg consumed in the tank circuit 101 is indicated by the parameter A.
Further, Gneg can be expressed by the oscillation frequency fosc, capacitance C and resistance Reff as indicated by the following equation (4).
Gneg=Reff(2xcfx80Cfosc)2xe2x80x83xe2x80x83(4)
As is clearly understood from the equation (2), the most important cause of the phase noise in the voltage controlled oscillator is the thermal noise caused by the parasitic resistance in the tank circuit 101, that is, Reff in the equation (2), but in the tank circuit 101 and amplifier 103 in which the thermal noise is sufficiently suppressed, noise caused by excessive current noise gives an extremely large influence. This is because excessive energy (current) is supplied for energy to be consumed in the tank circuit 101 and amplifier 103. As indicated by the equation (4), energy consumed in the tank circuit 101 and amplifier 103 is changed according to the oscillation frequency generated by the tank circuit 101.
It is also clearly understood from the equation (2) that output oscillation of the amplifier 103 also gives a large influence to the phase noise of the voltage controlled oscillator. Generally, the output amplitude and amplification factor of a high-frequency amplifier depend on the parasitic capacitance of the whole elements constructing the high-frequency amplifier.
In the conventional voltage controlled oscillator with the above construction, the variable capacitance diodes D1 and D2 are used to change the oscillation frequency, but the capacitance C in the equation (1) is required to be made larger as the oscillation frequency becomes lower. Therefore, the amplifier 103 is operated in a state in which the parasitic capacitance is extremely large as viewed from the amplifier side.
With the above fact taken into consideration, in a voltage controlled oscillator designed based on a certain oscillation frequency, the output amplitude thereof becomes smaller and the phase noise will be increased when the voltage controlled oscillator is used at a frequency lower;than the designed oscillation frequency.
Further, in a voltage controlled oscillator designed based on a certain oscillation frequency, an influence by excessive current noise becomes larger and the phase noise will be also increased when the voltage controlled oscillator is used at a frequency higher than the designed oscillation frequency.
FIG. 2 is a diagram showing the oscillation frequency dependency characteristic of the phase noise in the voltage controlled oscillator of FIG. 1. At the oscillation frequency of approximately 1235 MHz, the phase noise of the voltage controlled oscillator exhibits the minimum value. However, in a low frequency range lower than the above frequency and in a high frequency range higher than the above frequency, the phase noise is gradually increased.
It is considered that the above phenomenon occurs due to the reason described before. That is, the whole phase noise is increased by the excessive current at frequencies higher than the oscillation frequency at which the phase noise exhibits the minimum value. On the other hand, the whole phase noise is increased by a reduction in the output amplitude at frequencies lower than the oscillation frequency at which the phase noise exhibits the minimum value. FIG. 3 is a diagram showing the oscillation frequency dependency characteristic of the output amplitude in the voltage controlled oscillator of FIG. 1. FIG. 3 shows that the output amplitude is lowered as the oscillation frequency becomes lower and it is clearly understood from the equation (2) that the phase noise becomes larger with a decrease in the oscillation frequency.
Therefore, it has been desired to realize a voltage controlled oscillator having a stable and sufficiently suppressed phase noise characteristic irrespective of the oscillation frequency.
An object of this invention is to provide a voltage controlled oscillator having a stable and sufficiently suppressed phase noise characteristic irrespective of the oscillation frequency.
In order to attain the above object, a voltage controlled oscillator according to a first aspect of this invention comprises an amplifier circuit having a positive feedback construction, configured to output a signal of an oscillation frequency corresponding to a control voltage supplied to a variable capacitance element of a tank circuit having an inductor and the variable capacitance element; and a variable current source configured to change an operation current supplied to the amplifier circuit according to the control voltage; wherein, when a certain oscillation frequency at which phase noise becomes minimum is set as a reference point, the variable current source increases the operation current supplied to the amplifier circuit as the oscillation frequency becomes lower than the certain oscillation frequency, or decreases the operation current supplied to the amplifier circuit as the oscillation frequency becomes higher than the certain oscillation frequency.
A voltage controlled oscillator according to a second aspect of this invention comprises a control terminal to which a control voltage in a preset voltage range is supplied from an exterior; a first current source configured to output a preset first current; a second current source supplied with the control voltage from the control terminal, configured to output a second current controlled by the control voltage; a current mirror circuit supplied with a third current obtained by adding the first and the second current, and configured to supply a fourth current which is a mirror current of the third current; an oscillation circuit into which the fourth current of the current mirror circuit flows, the amplifier circuit including two variable capacitance diodes whose cathodes are connected together and supplied with the control voltage, an inductor connected between two anodes of the two variable capacitance diodes and cooperating with the variable capacitance diodes to oscillate at a frequency controlled by the control voltage, and at least one positive feedback circuit which is connected between the two anodes of the two variable capacitance diodes and into which the fourth current of the current mirror circuit flows; and two output terminals respectively connected to the anodes of the two variable capacitance diodes.
A voltage controlled oscillator according to a third aspect of this invention comprises a control terminal to which a control voltage in a preset voltage range is supplied from an exterior; a first current source into which a preset first current flows; a second current source which is supplied with the control voltage from the control terminal and into which a second current controlled by the control voltage flows; a current mirror circuit supplied with a third current obtained by adding the first and the second current and configured to supply a fourth current which is a mirror current of the third current; an oscillation circuit into which the fourth current of the mirror circuit flows, the oscillation circuit including two variable capacitance diodes, anodes of which are connected together and supplied with the control voltage, an inductor connected between two cathodes of the two variable capacitance diodes and cooperating with the variable capacitance diodes to oscillate at a frequency controlled by the control voltage, and at least one positive feedback circuit which is connected between the two cathodes of the two variable capacitance diodes and into which the fourth current of the current mirror circuit flows; and two output terminals respectively connected to the cathodes of the two variable capacitance diodes.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.